(modified part is underlined) The Drosophila RNase ZL (dRNaseZ) gene encodes a protein with homologs in all living organisms: bacteria, archaea, and eukarya. All RNase ZL proteins tested so far were found to possess endoribonuclease activity, which is responsible for the removal of a 3' trailer from pre-tRNA. Given that tRNA 3' endonucleolytic activity has been delineated using in vitro, bacterial and cell culture models, it's relevance to RNase ZL function in vivo has yet to be established. Recent studies suggested that RNase ZL might play critical roles in various biological events, including development, disease, cell proliferation, and mitochondrial biogenesis. However, no molecular networks that include RNase ZL have been identified and characterized so far, due in part to a very limited number of model systems studied in vivo. Human ELAC2, a gene that encodes the RNase ZL homolog, has been associated with mitochondria-related diseases and elevated risk of prostate cancer. However, the role of ELAC2 in these processes has not been definitively established. It is not known whether and which of these functions involve RNase ZL endonucleolytic activity. Our long-term objective is to identify developmental events and physiological processes in which RNase ZL is involved, and determine how it contributes to the control of these processes. Previously, we showed that dRNaseZ is an essential gene as its knockout impairs cell growth and proliferation. Based on the preliminary data, we suggest several hypotheses connecting the dRNaseZ function with cell growth and proliferation that will be tested through specific aims formulated in the proposal. First, by studying a collection of EMS-generated mutants we will identify alleles that uncover dRNaseZ functions independent of its role in tRNA processing. Second, using P element mediated transformation we will generate transgenic lines uncoupling nuclear and mitochondrial activities of dRNaseZ. We will test whether dRNaseZ is required to support the integrity of mtDNA and the expression of mitochondrial genes. And finally, we will use a genetic mosaic approach and ectopic expression of aberrant tRNA-precursor to study the role of dRNaseZ in protein synthesis machinery. The knowledge that we expect to gain from Drosophila RNase ZL studies will help better illuminate the possible contribution of its human counterpart to different human disease pathologies.